THE FOCI OF LENSES 13 



focus of a converging lens, the rays are brought to a focus beyond 

 the principal focus on the other side of the lens. The nearer the 

 radiant is to the principal focus, the farther away will be its conjugate 

 focus from the other principal focus. In other words, there are two 

 points in the axis such that if the object is one point its focus will 

 be the other ; these are reciprocal one to the other. These points, 



FIG. 11. Biconvex, plano-convex, 

 and converging meniscus lenses. 

 (From the ' Forces of Nature.') 



FIG. 12. Biconcave, plano-concave, 

 and diverging meniscus lenses. 

 (From the ' Forces of Nature.') 



the focal distances of which can always be calculated, are known as 

 conjugate foci. 



Should the radiant be at a distance from the principal focus equal 

 to the focal length of the lens (i.e. twice the focal length from the 

 lens), then its conjugate will be at the same distance from the focus 



FIG. 13. A radiant at the principal focus of a biconvex tens makes the refracted 



rays parallel. 



FIG. 14. A radiant placed beyond the principal focus causes rays to converge 

 beyond the principal focus on the other side of the lens. 



on the other side of the lens (i.e. twice the focal length from the lens). 

 In other words, when the object and its image are equidistant 011 

 either side of the lens, they are equal to each other in size, and 

 are four times the focal length of the lens apart. 



